This invention relates to a magnetoinductive flowmeter for moving fluids, incorporating a measuring tube, an electromagnet containing a field coil and serving to generate a magnetic field that extends at least essentially perpendicular to the axis of the measuring tube, a power supply serving to feed a switched DC field current to the field coil, at least two measuring electrodes positioned along a connecting line that extends at least essentially perpendicular to the axis of the measuring tube and to the direction of the magnetic field, and a detection circuit serving to analyze the measuring voltage of the measuring electrodes and to generate a flow signal; preferably also a current regulator controlling the field current and/or a controller controlling or regulating the voltage at the field coil of the electromagnet, as well as preferably a commutating or changeover bridge on the input side of the field coil of the electromagnet.
The fundamental principle of the magneto inductive flowmeter for moving fluids goes all the way back to Faraday who in 1832 postulated that the principle of electrody-namic induction could be employed for measuring flow rates. According to Faraday""s law of induction, a moving fluid which contains charge carriers and flows through a magnetic field will produce an electrical field intensity perpendicular to the flow direction and to the magnetic field. Magnetoinductive flowmeters operate on the basis of that law whereby a magnet, usually consisting of two field coils, generates a magnetic field, perpendicular to the direction of flow thin in the measuring tube. Within the magnetic field, each volume element of the fluid traveling through the magnetic field contributes, as a function of the field intensity building up in it, to the measuring voltage collected via the measuring electrodes. In conventional magnetoinductive flowmeters, the measuring electrodes are configured either for direct electrical or for capacitive connection with the moving fluid. A particular feature of magnetoinductive flowmeters is the proportionality between the measuring voltage and the flow rate of the fluid as measured across the diameter of the tube, i.e. between the measuring voltage and the volumetric flow.
Originally, industrial magnetoinductive flowmeters employed an alternating magnetic field. For reasons of cost economy, the electromagnet serving to produce the magnetic field was connected to the existing AC line voltage, generating essentially sinusoidal intensity changes. In these AC-operated magnetoinductive flowmeters, any line-voltage fluctuations would be directly transmitted to the electromagnet with a corresponding impact on the measuring accuracy of the magnetoinductive flowmeter. In order to still be able to obtain acceptably accurate measurements, magnetoinductive flowmeters employing an alternating magnetic field have been equipped with reference coils on the electromagnet, with the voltages induced in these reference coils being used to correct the measuring voltages at the measuring electrodes.
The problem with conventional magnetoinductive flowmeters operating with an alternating magnetic field is that the measuring voltage at the measuring electrodes is necessarily of the same frequency as the AC power mains. Moreover, in practice the measuring electrodes are additionally subjected to line-frequency interference voltages caused, for instance, by axial electric currents in the moving fluid, themselves caused, for example, by the grounding of pumps in the adjoining pipelines; these interference voltages are then superposed on the measuring voltages generated according to Faraday""s law. This is a drawback in that it makes it difficult at best to obtain good measuring accuracy with magnetoinductive flowmeters employing an alternating magnetic field, substantially limiting their suitability for industrial use.
Since the mid-nineteen seventies there has been a growing trend toward the use of magnetoinductive flowmeters employing a switched DC magnetic field, i.e. the field current used is a polarity-switched direct current. These flowmeter designs avoid many of the problems encountered with magnetoinductive flowmeters working with alternating magnetic fields and can thus yield very high measuring accuracy levels to within one per mil.
Magnetoinductive flowmeters employing a switched DC magnetic field benefit from the fact that the magnetic field is constant during the measuring phase. No magnetic field, however, can be switched from one stare to another in zero time. That is an unescapable fact since the magnetic field contains energy which must first be extracted from the magnetic field and then reintroduced for the reverse polarity state (ref. FIG. 3 of the German patent disclosure 197 13 751). Consequently, there will be switchover phases during which the magnetic field is not constant and accurate measurements are therefore not possible. These switchover phases are especially bothersome for two reasons. First, measurements cannot be made during the switchover phase which constitutes dead time. Second, it is desirable in view of the interferences, caused as a function of the reciprocal value of the field frequency and emanating from the measuring electrodes, to work with as high a field frequency as possible, yet the field frequency is limited by the duration of the switchover phases.
It is therefore the objective of this invention to design and further improve the magnetoinductive flowmeter, discussed above in some detail, in such fashion that the aforementioned system-dependent switchover phases are shorter than they have been in conventional magnetoinductive flowmeters, thus allowing the field frequency, i.e. the frequency at which the DC magnetic field is switched, to be increased.
The magnetoinductive flowmeter according to this invention which solves the problems mentioned, is initially and fundamentally characterized in that it incorporates an additional power source, by means of which a supplementary current can be injected in the field coil of the electromagnet at the beginning of each half-wave of the field current present in the form of a switched direct current.
So far, mention has been made of only one electromagnet and one field coil of the electromagnet. That takes into account that there is usually one more component provided with the electromagnet, apart from the field coil, often in the form of a core and often featuring a pole shoe, which component in the case of a predetermined magnetic field intensity increases the magnetic induction and the magnetic flux. From the functional perspective, however, one electromagnet requires only one field coil, i.e. a field coil by itself can constitute an electromagnet. Therefore, the following description will occasionally refer to the field coil only rather than to the field coil of the electromagnet or to the electromagnet per se.
Magnetoinductive flowmeters of the type discussed generally incorporate two electromagnets, each with a field coil. From the functional perspective, only one electromagnet, i.e. one field coil, is necessary. Where in typical fashion two electromagnets are used, each with a field coil, the field coils are usually connected in series. The following, however, will in many cases refer to only one electromagnet or the field coil of one electromagnet or simply one field coil. Whether two field coils are used or only one has no bearing on that which is significant in terms of the objective of, and solution provided, by this invention.
When a direct current is fed to a field coil having an inductance L and a resistance R, the direct current flowing through the field coil will increase within a time constant that is determined by the ratio between the inductance L and the resistance R. If a field coil has a predetermined inductance L, a desirably short time constant would be attainable by means of a large resistance R. However, the resistance R of a field coil is intrinsically undesirable since it uses up electrical power or electrical energy, a fact which is functionally unnecessary and therefore not wanted. It follows that a short time constant, however desirable, cannot be obtained by selecting a relatively large resistance R for the field coil.
The steady state to be achieved, i.e. the field current setpoint IS, requires only the voltage U that is determined by Ohm""s law; the necessary voltage U is the product derived from the field current setpoint value Is and the resistance R of the field coil. Existing technology provides for quicker attainment of the setpoint value IS by feeding to the field coil, at the point of the switchover, a voltage potential UU (UU=voltage at the point of polarity switching), which voltage potential is greater than the fixed voltage U needed for attaining the field current setpoint value IS. At that juncture, steps must be taken whereby, upon reaching the field current setpoint value IS, the field current I is prevented from increasing further, since without any such steps curtailing the field current I, the latter would rise to the level resulting from the voltage UU and the resistance R of the field coil.
Conventional magnetoinductive flowmeters thus require a current regulator holding the field current I at the field-current setpoint value IS and a controller regulating or controlling the voltage U at the field coil of the electromagnet. If only a current regulator were used, that current regulator would cons lume the electric power, i.e. the electric energy derived from the product of the difference between the voltage UU and the voltage U on the one hand and the field current setpoint value IS on the other. Additionally employing a controller which controls or regulates the voltage U at the field coil of the electromagnet ensures that substantially less electrical power or energy is used up in the current regulator.
What has been accomplished in conjunction with the provision for an xe2x80x9cincreased voltage UU at the time of the switchoverxe2x80x9d and, associated therewith, by employing a current regulator and perhaps additionally a controller is consistent with the current state of the art. What this invention provides is an additional power source, in addition to or in lieu of the provisions explained above, by means of which it is possible, immediately at the beginning of every half-wave of the field current I obtained in the form of a switched direct current, to injectxe2x80x94for a brief moment onlyxe2x80x94a supplementary current IZ into the field coil. This additional current IZ, i.e. the energy associated therewith, will relatively quickly withdraw more of the residual energy that is still present in the magnetic field prior to the switchover, thus permitting a substantial shortening of the switchover phase, i.e. a reduction of the time between the switchover point and the attainment of the field current setpoint value IS.